While considerable progress has been made in understanding properties relevant to the development of nano-electronics, integration of atomic, molecular or nano-scale entities into operational circuitry has been difficult [1-12]. A quantum dot is typically comprised of a small cluster of atoms or molecules that exhibit quantized levels that are separated by energies intermediate to those in the bulk material and isolated single atoms or molecules. As quantum dots are often formed in place with spatial specificity within a larger circuit and specifically addressed, quantum dots represent the best demonstration to date of how digital circuitry can benefit from molecular electronics [13-17].
One goal that has eluded the field is the creation of multi-quantum dot ensembles to embody the Quantum Cellular Automata (QCA) scheme proposed by Lent and coworkers in 1993 [18]. This new paradigm for computing is based upon “cells” of tunnel coupled quantum dots, and electrostatic interaction between adjacent cells. Such assemblies are predicted to transmit binary information and perform computations at extremely low energy cost [18, 19]. The prototypical QCA cell is comprised of 4 quantum dots arranged as a square. Local electrode control provides a net cell charge of 2 electrons, of two degenerate, antipodal ground state electronic configurations. Local electrostatic electrodes break symmetry and cause the QCA unit to occupy one or the other of the diagonal 2 electron states. These states can be mapped as logic levels “0” and “1”. By creating a sequence of cells, inter-cell electrostatic coupling allows binary information to be conveyed from one point to another. Elaborations of the QCA scheme allow full implementation of logic for computation while still retaining the conventions of binary digital computing. Retention of a binary computing scheme in QCA affords the benefit of allowing existing software to be run on an energy efficient QCA device.
In its simplest implementation, 4 quantum dot electrostatic QCA requires no quiescent current. The QCA scheme is “edge driven”, that is, the logic inputs are also the power sources [20]. QCA cells formed with quantum dots having dimensions on the order of tens of nanometers [19, 21], and a coupled 3 cell quantum wire [12] have been achieved. QCA-like processes were also observed in serial triple quantum dot systems [23]. These efforts to form a QCA device have limited implications in routine computational schemes owing to the fact that temperatures in the milli-Kelvin range are required to prevent scrambling of states [20]. Local, unintended fixed, or occasionally changing charges on the quantum dots, or spurious defect charges on a quantum dot make local electrostatic tuning to achieve 2 electron filling both necessary and challenging [20]. Cryogenic conditions are also required for an approach based upon implanted dopants [24].
A qubit is a quantized information system controllably changed between two logic levels of “0” and “1” and is modeled as a two-dimensional complex vector space. Unlike digital logic element, qubits are preferably constructed to exhibit quantum entanglement that is a manifestation of perturbations and tunnel coupling between proximal elements. The qubit entanglement term affords an infinite set of superposition values intermediate between “0” and “1” levels that can render computation of many multi-variable dynamic events tractable. As with QCA devices, electronic state changing qubits have not been able to operate at ambient temperatures.
An atomistic quantum dot is based on a single atom or molecule and has a simplified set of orbital energy levels that are beneficial in terms of a reduced dimensionality and being less susceptible to unintended fixed, or sporadic charging relative to nanoparticle quantum dots. As a result of these and other expected attributes, molecular QCA cells have been envisioned with the goal of room temperature operation [25]. Difficulties associated with localization and controlled addressing of molecular quantum dots has prevented molecular QCA cells from being produced. Similar problems exist for electronic state transition based qubits.
Thus, there exists a need for an electronically addressable molecular or atomic quantum dot with operational capabilities at temperatures above milli-Kelvin. There further exists a need for QCA and qubit devices containing ordered arrangements of molecular or atomic quantum dots.